The generator stator core is the largest monobloc component in a turbine-generator set. The stator core comprises thousands of thin steel laminations horizontally stacked and clamped together to form a cylindrical stator core disposed within a generator frame. Each lamination defines a central opening and thus when stacked, an axial opening extends through the core. The laminations are held together by a plurality of axial through-bolts that extend from end-to-end through the core.
A rotor is disposed within the central opening and mechanically rotated by a rotating turbine. The rotor is responsive to an electrical current such that rotation generates electric current in stator windings. The stator current is supplied to electrical loads through a transmission and distribution system.
Steady-state and transient forces generated during normal operation and transient conditions impose substantial forces on the stator core. These forces can also distort the core geometric shape, cause the laminations to vibrate, and damage the core, rotor and/or frame. Also, mechanical fatigue caused by these forces can lead to premature failure of the generator.
According to one prior art technique, the individual laminations, or a plurality of laminations in the form of a donut, engage keybars disposed on an inside surface of the generator frame. The keybars are rod-like or bar-like members that extend a length of the frame. An outwardly-facing surface of each keybar is attached to internal frame structures (e.g., frame rings). An inwardly-facing surface of each keybar comprises a projection that engages axial grooves in the outer circumference of the laminations (donuts).
Prior art FIG. 1 is a cutaway view of a prior art generator frame 2 prior to insertion of the stator core (i.e., stator core laminations or donuts). Keybars 6 having a dovetail cross-section extend the axial length of the frame 2 and are attached to generator frame support rings 4 via a transition adapter plate 5 (see prior art FIG. 2). The support rings 4 are in turn welded to the generator frame 2. The generator frame 2 is fixed to a stable support such as the floor of a power plant (not shown).
The laminations and keybars are engaged by mating complementary notches as shown in FIG. 2. A plurality of notches 12 are defined in a circumferential surface of a lamination 10. The notches 12 match a complementary profile of the keybars 6. When a plurality of laminations 10 is stacked horizontally the aligned notches form an axial groove, with several axial grooves formed in a circumference of the core. To assemble the stator core, the laminations 10 are slid onto the keybars 6. Since the keybars 6 are affixed to the generator fame 2 and the frame is attached to the power plant floor, the core and its constituent laminations are stabilized.
Since the stator core vibrates during operation, with large magnitude vibrations occurring during power system transients, it is critical for the keybars 6 to be rigidly affixed to both the laminations 10 and to the generator frame 2.
Prior art FIGS. 3 and 4 illustrate structural components for both maintaining circumferential alignment of the laminations and attaching the stator core to the generator frame. A stator core 21 comprises a plurality of horizontally-stacked module assemblies 20, each module assembly comprising a plurality of laminations. Each lamination module assembly 20 further defines a plurality of grooves 23 (see FIG. 4) formed in a circumferential surface thereof by the alignment of notches formed in the circumferential surface of each lamination. An end plate is disposed at each end of the core 21; only one end plate 63 illustrated in FIG. 3.
Each lamination defines a central opening for receiving stator windings. Thus each module assembly 20 and the stator core 21 also define the central opening for receiving the stator windings.
Through bolts 44A and mating nuts 44B (shown on only one end of the core 21 in FIG. 3) extend a length of the core 21 and cooperate to exert inwardly-directed axial clamping forces on the end plates and the lamination module assemblies 20.
A keybar 50 is disposed within each groove 23 of the lamination module assemblies 20, with a relatively tight fit clearance between the keybar and the groove. This tight fit prevents movement of the module assemblies 20 in a circumferential direction. Each keybar 50 comprises a threaded stud terminal portion 50A for receiving a mating nut 50B that is urged against the end plates 63 to maintain the circumferential alignment of the module assemblies 20. The keybar 50 extends the full axial length of the core 21.
According to this embodiment, to attach the stator core 21 to the generator frame a first plurality of axially aligned keybar attachment assemblies 60 are disposed between and attached to two adjacent keybars 50 as shown in FIG. 3. A second plurality of axially aligned keybar attachment assemblies (not shown in FIG. 3) are disposed about 180 circumferential degrees from the first plurality of keybar attachment assemblies.
The cross-sectional view of FIG. 4 illustrates a key bar attachment assembly 60 and a keybar attachment assembly 70 disposed at about 180 circumferential degrees apart. The keybar attachment assemblies 60 and 70 are attached to the keybars 50 by a fastener such as a bolt or screw 85. Each keybar attachment assembly 60/70 spans between two adjacent keybars 50 as illustrated in FIG. 3.
Each keybar attachment assembly 60/70 further comprises a groove 60A/70A for receiving a spring bar 86/88. The spring bars 86/88 extend the axial length of the core 21 and each is welded to frame support rings, such as the support rings 4 illustrated in FIG. 1. The spring bars 86/88 are affixed within the grooves 60A/70A by a weldment or a fastener. This arrangement of structural components rigidly attaches the stator core 21 to the generator frame.
FIG. 5 is a partial perspective view illustrating the components associated with another technique for attaching the stator core to the generator frame. A bracket 90 comprises a notched portion 91 and a base portion 92. A keybar 94 is received within a groove of a block 93. Fasteners 96 attach the base portion 91 to the block 93 and to the keybars 94. An axial spring bar 98 is disposed within a notch of the notched portion 91 and welded to the frame rings of the generator core. In one embodiment the frame rings define notches therein for receiving the spring bars 98.
Typically, a first set of three brackets 90 are attached at three axially aligned locations along the stator core. A second set of three brackets 90 are attached at three axially aligned locations separated by 180 degrees from the location of the first set of three notched brackets.
Combinations of the various core-to-frame-attachment techniques described herein can also be utilized to maintain alignment of the stator core laminations and to fixedly attach the core to the generator frame.
Unfortunately, to accommodate the various different core-to-frame attachment techniques, numerous and different attachment hardware components and weldments are required. Additionally, this technique requires use of multiple high-strength engineered components to provide the necessary precision fit between mating components and to adequately transfer the dead weight and operating loads (caused by both steady-state and transient forces) from the stator core to the generator frame and then to the generator foundation.
In particular as related to the embodiment of FIG. 5, the forces are substantial in the region where the bracket 90 is attached to the keybar 94 via the block 93, especially during fault conditions. The exerted forces and loads produced by these attachment techniques are also difficult to quantitatively and accurately analyze. Additionally, a time-consuming assembly process is necessary to assemble the components.
According to yet another attachment technique, a plurality of steel bands is attached around the girth of the stator core. Tabs extending outwardly from the surface of the stator core are welded to the steel bands. The tabs are then attached to flex springs in the stator frame to complete the core-to-frame-to-foundation attachment assembly. Typically the tab is welded to the flex spring.
Disadvantageously, this last described attachment technique also requires numerous components. It is difficult and time-consuming to pull the steel bands around the core girth then weld them in place. After this step has been completed, the tab is welded to the flex spring, which again requires significant and relatively flawless welding.
Those skilled in the art recognize that given the wide variety of generator styles, sizes and ratings, there are a wide variety of generator frame constructions, stator constructions and core attachment components. It is always desired to utilize technically sound structural components for attaching the core to the frame. These components must provide the necessary structural rigidity (e.g., to limit frame vibrations) during normal operation and during fault conditions. These structural components must also permit the laminations (or donuts comprising a plurality of laminations) to be easily secured to the generator frame. Of course it is also desired to reduce the time and cost expended to manufacture these structural components and reduce the time required and the complexity of the process for attaching the core to the frame.